1. Field of the Invention
The present invention is directed generally to wireless communication systems, and more specifically, to transmission methods for acknowledgement signals including the application of transmission diversity.
2. Description of the Art
A communication system includes a DownLink (DL), supporting transmissions of signals from a base station (Node B) to User Equipments (UEs), and an UpLink (UL), supporting transmissions of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, and the like. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
The UL signals from a UE include data signals, carrying the information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The UL control signals include acknowledgement signals associated with the application of a Hybrid Automatic Repeat reQuest (HARQ) process and are in response to the correct or incorrect, respectively, reception of data Transport Blocks (TBs) by the UE. UL control signals can be transmitted separately from data signals in a Physical Uplink Control CHannel (PUCCH) or, they can be transmitted together with data signals, in a Physical Uplink Shared CHannel (PUSCH) over a Transmission Time Interval (TTI). The UE receives TBs from the Node B through a Physical Downlink Shared CHannel (PDSCH) and the Node B schedules transmission of the TBs in the PDSCH or transmission of the TBs from the UE in the PUSCH through Downlink Control Information (DCI) formats transmitted in a Physical Downlink Control CHannel (PDCCH).
A PUCCH structure for the HARQ ACKnowledgement (HARQ-ACK) signal transmission in the UL TTI, which, for simplicity, is assumed to include one sub-frame, is illustrated in FIG. 1. The sub-frame 110 includes two slots. Each slot 120 includes NsymbUL symbols for the transmission of HARQ-ACK signals 130, or of RS 140 which enable coherent demodulation of the HARQ-ACK signals. Each symbol further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in the first slot may be at a different part of the operating BandWidth (BW) than the second slot to provide frequency diversity. The operating BW is assumed to consist of frequency resource units, which will be referred to as Resource Blocks (RBs). Each RB is further assumed to include NscRB sub-carriers, or Resource Elements (REs), and a UE transmits HARQ-ACK signals and RS over one RB 150.
A structure for the HARQ-ACK signal transmission in one slot of the PUCCH is illustrated in FIG. 2. The transmission in the other slot is assumed to effectively have the same structure. The HARQ-ACK bits b 210 modulate 220 a “Constant Amplitude Zero Auto-Correlation (CAZAC)” sequence 230, using, for example, Binary Phase Shift Keying (BPSK) or Quaternary Phase Shift Keying (QPSK) modulation, which is then transmitted after performing an Inverse Fast Frequency Transform (IFFT) as it is next described. The RS 240 is transmitted through the non-modulated CAZAC sequence.
An example of CAZAC sequences is given by Equation (1).
                                          c            k                    ⁡                      (            n            )                          =                  exp          ⁡                      [                                                            j2π                  ⁢                                                                          ⁢                  k                                L                            ⁢                              (                                  n                  +                                      n                    ⁢                                                                  n                        +                        1                                            2                                                                      )                                      ]                                              (        1        )            
In Equation (1), L is the length of the CAZAC sequence, n is the index of an element of the sequence n={0, 1, . . . , L−1}, and k is the index of the sequence. If L is a prime number, there are L−1 distinct sequences which are defined as k ranges in {0, 1, . . . , L−1}. If an RB is comprised of an even number of REs, such as, for example, NscRB=12, CAZAC sequences with an even length can be directly generated through a computer search for sequences satisfying the CAZAC properties.
FIG. 3 illustrates a UE transmitter structure for the HARQ-ACK signal in the PUCCH. The frequency-domain version of a computer generated CAZAC sequence 310 is assumed. The first RB and second RB are selected 320 for transmission 330 of the CAZAC sequence in the first and second slots, respectively, an IFFT is performed 340, and a Cyclic Shift (CS), as it is subsequently described, applies to the output 350. Finally, the CP 360 and filtering 370 are applied to the transmitted signal 380. A UE is assumed to apply zero padding in REs that are not used for its signal transmission and in guard REs (not shown). Moreover, for brevity, additional transmitter circuitry such as a digital-to-analog converter, analog filters, amplifiers, and transmitter antennas as they are known in the art, are not shown.
The reverse (complementary) transmitter functions are performed by the Node B for the HARQ-ACK signal reception in the PUCCH. This is illustrated in FIG. 4, where the reverse operations of those in FIG. 3 apply. An antenna receives the RF analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) the received digital signal 410 is filtered 420 and the CP is removed 430. Subsequently, the CS is restored 440, a Fast Fourier Transform (FFT) 450 is applied, the first RB and the second RB of the signal transmission 460 in the first slot and in the second slot, respectively, are selected 465, and the signal is correlated 470 with the replica 480 of the CAZAC sequence. The output 490 can then be passed to a channel estimation unit, such as a time-frequency interpolator, in the case of RS, or to detect the transmitted HARQ-ACK information.
Different CSs of the same CAZAC sequence provide orthogonal CAZAC sequences and can be assigned to different UEs for orthogonal multiplexing of signal transmissions in the same PUCCH RB. This principle is illustrated in FIG. 5. In order for the multiple CAZAC sequences 510, 530, 550, 570 generated respectively from the multiple CSs 520, 540, 560, 580 of the same root CAZAC sequence to be orthogonal, the CS value Δ590 should exceed the channel propagation delay spread D (including time uncertainty errors and filter spillover effects). If TS is the symbol duration, the number of such CSs is equal to the mathematical floor, i.e., rounding down, of the ratio TS/D.
In addition to orthogonal multiplexing of HARQ-ACK signal transmissions from different UEs in the same RB using different CS of a CAZAC sequence, orthogonal multiplexing can also be achieved in the time domain using Orthogonal Covering Codes (OCCs). For example, in FIG. 2, the HARQ-ACK signal can be modulated by a length-4 OCC, such as a Walsh-Hadamard (WH) OCC, while the RS can be modulated by a length-3 OCC, such as a DFT OCC (not shown for brevity). In this manner, the PUCCH multiplexing capacity is increased by a factor of 3 (determined by the OCC with the smaller length). The sets of WH OCCs, {W0, W1, W2, W3}, and DFT OCCs, {D0, D1, D2}, are:
            [                                                  W              0                                                                          W              1                                                                          W              2                                                                          W              3                                          ]        =          [                                    1                                1                                1                                1                                                1                                              -              1                                            1                                              -              1                                                            1                                1                                              -              1                                                          -              1                                                            1                                              -              1                                                          -              1                                            1                              ]        ,            [                                                  D              0                                                                          D              1                                                                          D              2                                          ]        =                  [                                            1                                      1                                      1                                                          1                                                      ⅇ                                                      -                    j2π                                    /                  3                                                                                    ⅇ                                                      -                    j4π                                    /                  3                                                                                        1                                                      ⅇ                                                      -                    j4π                                    /                  3                                                                                    ⅇ                                                      -                    j2π                                    /                  3                                                                    ]            .      
Table 1 illustrates a mapping for a PUCCH resource nPUCCH, used for HARQ-ACK signal transmission, to an OCC noc and a CS α assuming a total of 12 CS of the CAZAC sequence per PUCCH symbol.
TABLE 1HARQ-ACK Resource Mapping to OC and CSOC noc for HARQ-ACK and for RSCS aW0, D0W1, D1W3, D20nPUCCH = 0nPUCCH = 121nPUCCH = 62nPUCCH = 1nPUCCH = 133nPUCCH = 74nPUCCH = 2nPUCCH = 145nPUCCH = 86nPUCCH = 3nPUCCH = 157nPUCCH = 98nPUCCH = 4nPUCCH = 169nPUCCH = 1010nPUCCH = 5nPUCCH = 1711nPUCCH = 11
The DCI formats are transmitted in elementary units, which are referred to as Control Channel Elements (CCEs). Each CCE consists of a number of REs and the UEs are informed of the total number of CCEs, NCCE, through the transmission of a Physical Control Format Indicator CHannel (PCFICH) by the Node B. For a Frequency Division Duplex (FDD) system and PDSCH transmission scheduled by a DCI format, the UE determines nPUCCH from the first CCE, nCCE, of the DCI format with the addition of an offset NPUCCH that is configured by higher layers (such as the Radio Resource Control (RRC) layer) and nPUCCH=nCCE+NPUCCH. For a Time Division Duplex (TDD) system, the determination of nPUCCH is more complex, as further discussed below, but the same mapping principle using the CCEs of the DCI format scheduling the corresponding PDSCH transmission applies.
In TDD systems, DL and UL transmissions occur in different sub-frames. For example, in a frame including 10 sub-frames, some sub-frames may be used for DL transmissions and some sub-frames may be used for UL transmissions.
FIG. 6 illustrates a 10 millisecond (ms) frame structure which includes two identical half-frames. Each 5 ms half-frame 610 is divided into 8 slots 620 and 3 special fields: a DL ParT Symbol (DwPTS) 630, a Guard Period (GP) 640, and an UL ParT Symbol (UpPTS) 650. The length of DwPTS+GP+UpPTS is one sub-frame (1 ms) 660. The DwPTS may be used for the transmission of synchronization signals from the Node B while the UpPTS may be used for the transmission of random access signals from UEs. The GP facilitates the transition between DL and UL transmissions by absorbing transient interference.
In TDD systems, the number of DL and UL sub-frames per frame can be different and multiple DL sub-frames may be associated with a single UL sub-frame. The association between the multiple DL sub-frames and the single UL sub-frame is that HARQ-ACK information generated in response to PDSCH transmissions in the multiple DL sub-frames needs to be conveyed in the single UL sub-frame.
A first method for a UE to convey HARQ-ACK information in a single UL sub-frame, in response to PDSCH transmissions in multiple DL sub-frames, is HARQ-ACK bundling where the UE sends a positive ACKnowledgement (ACK) only if all TBs in the respective PDSCHs are received correctly and sends a Negative ACKnowledgement (NACK) in other cases. Therefore, HARQ-ACK bundling results in unnecessary retransmissions and reduced DL throughput as a NACK is sent even when the UE correctly receives some, but not all, TBs in the respective PDSCHs. Another method for a UE to convey HARQ-ACK information in a single UL sub-frame, in response to TBs in the respective PDSCHs in multiple DL sub-frames, is HARQ-ACK multiplexing, which is based on PUCCH resource selection for the HARQ-ACK signal transmission as it is subsequently described. The invention primarily focuses on HARQ-ACK multiplexing.
In one embodiment, there could be 1, 2, 3, 4 or 9 DL sub-frames associated with 1 UL sub-frame. Therefore, assuming that a UE receives a maximum of 2 TBs per PDSCH in a DL sub-frame, the number of HARQ-ACK bits to be transmitted in the UL sub-frame can be 1, 2, 3, 4, 6, 8, 9 or 18. Supporting such a dynamic range of number of HARQ-ACK bits is typically not desirable as it is difficult to ensure, at the Node B, the required detection reliability, including the absence of an expected HARQ-ACK signal transmission due to a missed DCI format for PDSCH transmission to the UE (referred to as DTX). To reduce the number of HARQ-ACK bits, bundling can be applied in the spatial domain, resulting in a single HARQ-ACK bit in the case of 2 TBs in a PDSCH. This reduces the number of possible HARQ-ACK bits in the UL sub-frame to 1, 2, 3, 4, or 9. Further bundling in the time domain can be applied to the case of 9 HARQ-ACK bits so that the maximum number is always reduced to 4 HARQ-ACK bits. HARQ-ACK multiplexing can then be used to transmit up to 4 HARQ-ACK bits.
With HARQ-ACK multiplexing, a UE conveys a HARQ-ACK value (ACK, NACK, or DTX) for each of the multiple DL sub-frames even if PDSCH transmission to that UE did not occur in all DL sub-frames. For example, if there are 4 DL sub-frames for which HARQ-ACK information needs to be transmitted in the same UL sub-frame, then, with HARQ-ACK multiplexing, the HARQ-ACK signal from a UE conveys HARQ-ACK information for each of the 4 DL sub-frames even if the PDSCH transmission to the UE occurs in less than 4 DL sub-frames.
Table 2 illustrates the HARQ-ACK multiplexing in the case in which the UE conveys HARQ-ACK information for 2 DL sub-frames in the same UL sub-frame (a HARQ-ACK state consists of 2 HARQ-ACK values). The UE selects one PUCCH resource, nPUCCH(0) or nPUCCH(1), and one QPSK constellation point (on a constellation diagram) for the transmission of the QPSK modulated HARQ-ACK signal depending on the HARQ-ACK information. Each PUCCH resource is determined from the first CCE of the DCI format for the respective PDSCH transmission in each of the 2 DL sub-frames.
TABLE 2HARQ-ACK Multiplexing for 2 DL Sub-FramesHARQ-ACK(0), HARQ-ACK(1)nPUCCHQPSKACK, ACKnPUCCH(1)1, 1ACK, NACK/DTXnPUCCH(0)0, 1NACK/DTX, ACKnPUCCH(1)0, 0NACK/DTX, NACKnPUCCH(1)1, 0NACK, DTXnPUCCH(0)1, 0DTX, DTXN/AN/A
FIG. 7 illustrates the HARQ-ACK signal transmission process in Table 2. If no DCI format is received by the UE, there is no HARQ-ACK signal transmission. If the UE receives a DCI format in the second DL sub-frame 702, it uses the respective first CCE to determine nPUCCH (1) 710 for the HARQ-ACK signal transmission having {NACK/DTX, ACK} 722, {ACK, ACK} 724, and {NACK/DTX, NACK} 726 as the possible HARQ-ACK states which are then mapped to QPSK constellation points. If the UE receives a DCI format only in the first DL sub-frame 704, it uses the respective first CCE to determine nPUCCH (0) 730 for the HARQ-ACK signal transmission having {ACK, NACK/DTX} 742, and {NACK, DTX} 744 as the possible HARQ-ACK states which are then mapped to QPSK constellation points.
Table 2 illustrates the HARQ-ACK multiplexing in the case in which the UE conveys HARQ-ACK information for 2 DL sub-frames in the same UL sub-frame (a HARQ-ACK state consists of 2 HARQ-ACK values). The UE selects one PUCCH resource, nPUCCH(0) or nPUCCH(1), and one QPSK constellation point for the transmission of the QPSK modulated HARQ-ACK signal depending on the HARQ-ACK information. Each PUCCH resource is determined from the first CCE of the DCI format for the respective PDSCH transmission in each of the 2 DL sub-frames.
Table 3 illustrates the HARQ-ACK multiplexing in the case in which the UE conveys HARQ-ACK information for 3 DL sub-frames in the same UL sub-frame (a HARQ-ACK state consists of 3 HARQ-ACK values). The UE selects one PUCCH resource, nPUCCH(0), or nPUCCH(1), or nPUCCH(2), and one QPSK constellation point for the transmission of the QPSK modulated HARQ-ACK signal, depending on the HARQ-ACK information. Each PUCCH resource is determined from the first CCE of the DCI format for the respective PDSCH transmission in each of the 3 DL sub-frames. Explicit DTX indication is possible through the inclusion in the DCI formats for PDSCH transmission of a Downlink Assignment Index (DAI) Information Element (IE) indicating the cumulative number of assigned PDSCH transmission(s) to the UE.
TABLE 3HARQ-ACK Multiplexing for 3 DL Sub-FramesEntryHARQ-ACK(0), HARQ-ACK(1),NumberHARQ-ACK(2)nPUCCHQPSK1ACK, ACK, ACKnPUCCH(2)1, 12ACK, ACK, NACK/DTXnPUCCH(1)1, 13ACK, NACK/DTX, ACKnPUCCH(0)1, 14ACK, NACK/DTX, NACK/DTXnPUCCH(0)0, 15NACK/DTX, ACK, ACKnPUCCH(2)1, 06NACK/DTX, ACK, NACK/DTXnPUCCH(1)0, 07NACK/DTX, NACK/DTX, ACKnPUCCH(2)0, 08DTX, DTX, NACKnPUCCH(2)0, 19DTX, NACK, NACK/DTXnPUCCH(1)1, 010NACK, NACK/DTX, NACK/DTXnPUCCH(0)1, 011DTX, DTX, DTXN/AN/A
Finally, Table 4 describes the HARQ-ACK multiplexing in case the UE conveys HARQ-ACK information for 4 DL sub-frames in the same UL sub-frame (a HARQ-ACK state consists of 3 HARQ-ACK values). The UE selects one PUCCH resource, nPUCCH(0), or nPUCCH(1), nPUCCH(2), or nPUCCH(3), and one QPSK constellation point for the transmission of the QPSK modulated HARQ-ACK signal depending on the HARQ-ACK information. Each PUCCH resource is determined from the first CCE of the DCI format for the respective PDSCH transmission in each of the 4 DL sub-frames.
TABLE 4HARQ-ACK Multiplexing for 4 DL Sub-FramesEntryHARQ-ACK(0), HARQ-ACK(1),NumberHARQ-ACK(2), HARQ-ACK(3)nPUCCHQPSK1ACK, ACK, ACK, ACKnPUCCH(1)1, 12ACK, ACK, ACK, NACK/DTXnPUCCH(1)1, 03NACK/DTX, NACK/DTX, NACK, DTXnPUCCH(2)1, 14ACK, ACK, NACK/DTX, ACKnPUCCH(1)1, 05NACK, DTX, DTX, DTXnPUCCH(0)1, 06ACK, ACK, NACK/DTX, NACK/DTXnPUCCH(1)1, 07ACK, NACK/DTX, ACK, ACKnPUCCH(3)0, 18NACK/DTX, NACK/DTX, NACK/DTX, NACKnPUCCH(3)1, 19ACK, NACK/DTX, ACK, NACK/DTXnPUCCH(2)0, 110ACK, NACK/DTX, NACK/DTX, ACKnPUCCH(0)0, 111ACK, NACK/DTX, NACK/DTX, NACK/DTXnPUCCH(0)1, 112NACK/DTX, ACK, ACK, ACKnPUCCH(3)0, 113NACK/DTX, NACK, DTX, DTXnPUCCH(1)0, 014NACK/DTX, ACK, ACK, NACK/DTXnPUCCH(2)1, 015NACK/DTX, ACK, NACK/DTX, ACKnPUCCH(3)1, 016NACK/DTX, ACK, NACK/DTX, NACK/DTXnPUCCH(1)0, 117NACK/DTX, NACK/DTX, ACK, ACKnPUCCH(3)0, 118NACK/DTX, NACK/DTX, ACK, NACK/DTXnPUCCH(2)0, 019NACK/DTX, NACK/DTX, NACK/DTX, ACKnPUCCH(3)0, 020DTX, DTX, DTX, DTXN/AN/A
The main drawback of the mapping in Table 4 is that several HARQ-ACK states are mapped to the same PUCCH resource and QPSK constellation point (i.e., they are overlapping). For example, 3 different HARQ-ACK states in Table 4 (entries 2, 4, and 6) are mapped to PUCCH resource nPUCCH(1) and QSPK constellation point {1, 0}. Similarly, 3 other HARQ-ACK states (entries 7, 12, and 17) are mapped to PUCCH resource nPUCCH(3) and QSPK constellation point {0, 1}. This overlap is unavoidable since the 20 HARQ-ACK states in Table 4 must be mapped to a maximum of 16 positions corresponding to 4 PUCCH resources and 4 QPSK constellation points.
A consequence of the overlapping HARQ-ACK states in Table 4 is loss of system throughput, as the Node B typically needs to assume that non-unique values correspond to NACK or DTX and perform HARQ retransmissions although the UE may have actually correctly received the TBs of the respective PDSCHs. If the Node B schedules PDSCH transmissions to a UE in the first and second sub-frames, it is generally unable to schedule PDSCH transmissions to the UE in the third or fourth sub-frames (entries 2, 4, and 6). Similarly, if the Node B schedules PDSCH transmissions to a UE in the third and fourth sub-frames, it is generally unable to schedule PDSCH transmissions to the UE in the first or second sub-frame (entries 7, 12, and 17). Therefore, the mapping in Table 4 should be improved to minimize or avoid the overlapping of HARQ-ACK states. Specific rules should also be defined for iteratively constructing mapping Tables as the number of HARQ-ACK states increases.
For a UE equipped with more than one transmitter antenna, Transmitter Diversity (TxD) can enhance the reliability of the received signal at the Node B by providing spatial diversity. For HARQ-ACK signal transmission, because of the OCC applied across PUCCH symbols and because of possible CS hopping across PUCCH symbols within a slot, the application of TxD methods using space-time coding is problematic. Conversely, Orthogonal Resource Transmission Diversity (ORTD), where each UE transmitter antenna uses a separate (orthogonal) PUCCH resource, can directly apply.
FIG. 8 illustrates the application of ORTD. The first UE transmitter antenna uses a first PUCCH resource 810, associated with the first CCE used to transmit the DCI format, and the second UE transmitter antenna uses a second PUCCH resource 820, which can be assumed to be associated with a second CCE used to transmit the DCI format. Both antennas transmit the same information, which is either an ACK 830 and 850, or a NACK 840 and 860.
Although ORTD requires additional PUCCH resources, a UE may often have available more than one orthogonal PUCCH resource for HARQ-ACK signal transmission. For example, when the DCI format scheduling the PDSCH transmission uses more than one CCE for its transmission, each CCE provides an orthogonal PUCCH resource for HARQ-ACK signal transmission. However, without additional mechanisms, such as a separate configuration of an additional orthogonal PUCCH resource for UEs applying ORTD, the use of ORTD for HARQ-ACK signal transmission is generally problematic as the DCI format scheduling the respective PDSCH transmission may consist of only one CCE and the next CCE may be the first CCE used for the transmission of another DCI format scheduling PDSCH transmission to another UE.
The HARQ-ACK multiplexing used for TDD systems can be extended for FDD systems using Carrier Aggregation (CA) where a UE receives multiple PDSCH transmissions in multiple DL cells in the same TTI. CA is fundamentally the parallelization of single-cell operation to multi-cell operation. For each PDSCH reception, the UE needs to convey to the Node B one HARQ-ACK value (ACK, NACK, or DTX) in case the PDSCH conveyed one TB and two HARQ-ACK values ({ACK, ACK}, {ACK, NACK}, {NACK, ACK}, {NACK, NACK} or DTX) in case the PDSCH conveyed two TBs.
Therefore, there is a need to enable ORTD for HARQ-ACK signal transmission with multiplexing by utilizing available CCEs used for transmission of respective DCI formats.
There is another need to optimize the use of PUCCH resources for HARQ-ACK signal transmission using multiplexing.
There is another need to minimize or avoid overlapping of HARQ-ACK states onto the same PUCCH resources or QPSK constellation points and to define iterative mapping rules as the number of HARQ-ACK states increases.
Finally, there is another need to support HARQ-ACK multiplexing for FDD systems using carrier aggregation.